The present invention relates to low NOx emitting burners which are compact, efficient to operate, and employ furnace gas recirculation inside the combustion chamber of the furnace to reduce NOx emissions.
Furnace emissions are of great concern because they significantly contribute to atmospheric pollution. A large source for NOx emissions is burners as used in large and small furnaces, including, for example, very large furnaces used for generating electric power with steam-operated turbines. It is well known that NOx emissions are reduced by lowering the temperature of the flame generated by the burner inside the furnace. Conventionally this has been attained by supplying the burner with excess air over what would be required to stoichiometrically fire the fuel, because the fuel must heat the additional air, which lowers the overall temperature of the flame and the furnace gases generated thereby.
Another approach to lowering NOx emissions is to mix the combustion air for the burner with flue gas going to the exhaust stack. This technique is called flue gas recirculation (FGR). Flue gas typically has a temperature in the range of between about 200° F. to 400° F. Recirculated flue gas lowers flame temperatures and NOx generation, but in excessive amounts causes flame instability and blowout.
Both of these approaches can be used individually or in combination. However, large amounts of FGR that might be necessary for reducing NOx substantially increase the overall volume of gas that must be transported through the burner and the furnace convection section. This in turn requires larger blowers and conduits, including the common windbox outside the front wall of a burner, to handle the increased combined mass of air and FGR with an elevated temperature that must be transported through the system. This increases initial installation costs as well as subsequent operation and maintenance costs due to the increased energy requirements of the blower, all of which is undesirable.
As disclosed in the above-referenced, copending application, high amounts of FGR that must be recirculated can be reduced by recirculating furnace gases internally of the combustion chamber. This has worked well in reducing NOx emissions and has the advantage that it reduces or eliminates additional energy to operate a larger blower to handle additional combustion air and/or recirculated flue gas. The main part of the burner disclosed in the copending application is a massive cylindrical tube which extends from the furnace wall. The spinner is mounted at the discharge end of this tube. The portion of the tube proximate the furnace wall includes openings through which furnace gases are aerodynamically driven by air and fuel gas jets inside the tube where the furnace gases are mixed with combustion air and fuel prior to the ignition of the mixture. However, this burner is susceptible to overheating and damage to the tube if fuel starts burning inside the confines of the tube. Conditions for the fuel burning inside the tube may happen when the overall incoming mixture of air, flue gas and fuel gas is insufficiently diluted with inert gases like FGR. Steering the operating regimes of the burner away from the flame burning inside also requires shifting more toward the discharge end of the tube that is usually not optimal for achieving the lowest NOx emissions.